For use in an engine of an automobile, for example, a drive-by-wire system (hereinafter referred to as "DBW") has been developed which is used for transmitting electric signals between an accelerator pedal and a throttle valve of the engine. In this DBW system, the accelerator pedal and the throttle valve are not mechanically connected to each other, and a virtual accelerator position (pseudo accelerator position) is determined based on the actual amount of depression of the accelerator pedal (actual accelerator position) and various other parameters,. The DBW system is able to control the throttle valve according to the virtual (pseudo) accelerator position, and may also be called "electronic throttle control device".
During an idling operation of the vehicle in which the accelerator pedal is not depressed (namely, the amount of depression of the accelerator pedal is lower than an infinitesimal value), for example, the DBW system is able to control the idle speed by finely adjusting the opening of the throttle valve. Also, the DBW system is able to set the pseudo accelerator position by correcting the actual accelerator position (the amount of depression of the pedal by the driver) according to the running state of the vehicle or operating state of the engine, and control the throttle valve based on this pseudo accelerator position, thereby to achieve an engine operation that gives the driver a good driving feeling.
As one type of internal combustion engines (generally, gasoline engines) using spark plugs for enabling spark ignition, in-cylinder fuel injection type spark ignition engines (hereinafter simply called "engine") in which a fuel is directly injected into each cylinder have been put in practical use. In this type of engine, the timing of fuel injection can be freely selected as desired, and the composition (air-fuel ratio) of an air-fuel mixture formed in a combustion chamber can be freely controlled. These advantageous features contribute to improvements in both of the fuel cost performance and output performance.
The in-cylinder fuel injection type spark ignition engine may operate in a first lean-burn mode (compression stroke injection mode) as one of combustion modes, in which the fuel is injected during a compression stroke, so that a fuel-lean, air-rich mixture (whose air fuel ratio is considerably larger than the stoichiometric ratio) undergoes startified charge combustion, to thus achieve an extreme lean-burn operation, assuring a significantly improved specific fuel consumption.
Needless to say, the in-cylinder fuel injection type spark ignition engine is also able to inject the fuel into a cylinder mainly during a suction or intake stroke, and burn an air-fuel mixture that has been mixed together before combustion. In this case, the fuel is directly injected into a combustion chamber within a cylinder, whereby most of the fuel injected in each combustion cycle can be surely burned in the same combustion cycle, to thus provide an improved engine output.
The above-described combustion operation with the pre-mixed fuel and air may be performed in one of combustion modes: 1) a second lean-burn mode in which the engine operates with a fuel-lean, air-rich mixture (whose air fuel ratio is larger than the stoichiometric ratio) though the mixture contains a smaller percentage of intake air than that formed in the first lean-burn mode, 2) stoichiometric operation mode (stoichiometric feedback operation mode) in which feedback control is performed based on information from an O.sub.2 sensor so that the air fuel ratio becomes substantially equal to the stoichiometric ratio, and 3) enrich operation mode (open-loop mode) in which the engine operates with a mixture having a high percentage of fuel (namely, a mixture whose air fuel ratio is smaller than the stoichiometric ratio).
Generally, if the required output of the engine is small, namely, if the engine speed is low and the load is small, the first lean-burn mode is established so as to reduce fuel consumption and improve fuel economy. As the engine speed and engine load increase, the operating mode of the engine is selected in the order of the second lean-burn mode, stoichiometric operation mode, and enrich operation mode.
When the engine operates in the extreme lean-burn mode (first lean-burn mode), an increased amount of air needs to be supplied to each combustion chamber so as to increase the air fuel ratio. In the first lean-burn mode, however, the engine operates in a region where the engine load is low, namely, the amount of depression of the accelerator pedal (difference between the current accelerator pedal position and its fully released position) is small, and therefore a desired air fuel ratio cannot be achieved if the opening of the throttle valve is controlled according to the amount of depression of the accelerator pedal.
A technique for dealing with the above problem has been developed, wherein an air bypass passage is provided which bypasses an intake passage having a throttle valve, and an electronic controlled valve (air bypass valve) is mounted in this air bypass passage. When the amount of intake air supplied to each combustion chamber is insufficient due to a small opening of the throttle valve controlled according to the accelerator position, the air bypass valve is opened depending upon a desired amount of intake air, so as to supply extra air into the combustion chamber.